“Ballooning” crab spiders spin silk parachutes, and take off after testing the wind with their legs

A new study in PLoS Biology by Moonsung Cho et al. (free pdf here; reference below) uncovers some of the mysteries of how spiders (in this case crab spiders) balloon. “Ballooning” is an amazing form of spider dispersal. The spiders, usually very young ones, climb up on some high spot like a blade of grass or a twig, and then emit long strands of silk from their spinnerets on the abdomen; those strands then catch the wind and carry the spiderlets for long distances—even hundreds of miles.

Why do they do this? There are a number of reasons mentioned by Cho et al. including:

Reducing cannibalism by fellow spiderlings

Reducing competition for local resources

Dispersing to new and more favorable sites

Searching for mates and food

According to the authors, ballooning spiders have traveled hundreds of kilometers this way, colonizing distant “oceanic” (volcanic) islands, and have even been seen as high as 4.5 km above sea level.

Here’s what ballooning looks like (this Nat. Geo. video mentions the new results):

Despite this well known phenomenon, a number of questions remained. How do they know if the wind is right? What kind of “sail” do they produce, and how do they do it?

The PLOS biology paper is long, and I’ll summarize just a few interesting results: these were taken from observations in nature, from wind-tunnel experiments, and from outdoor experiments in which spiders were put atop artificial platforms that emitted a powder that showed the wind speed and direction.

First, the spiders actually test the wind conditions before they take off by raising one or two front legs into the air—just like humans test the wind direction by wetting a finger and raising it. They keep the legs up for about 6-8 seconds, thereby seeing if conditions are right for takeoff. If they are—and that means the winds are less than about 3 meters per second—they then turn their body around, get on “tiptoe”, raising their butts into the air, and emit a series of silken threads, several meters long, to form a triangular parachute. Here’s a figure from the paper showing the wind-testing, body rotation, and tiptoe posture. (All captions come from the original paper.)

(From paper): Sequence of active sensing motion with front leg (leg I) (negative images). (A) The spider first senses the condition of the wind current only through sensory hairs on its legs. (B) Then, if the condition seemed appropriate, the spider sensed more actively by raising leg I and keeping this pose for 8 sec. (C) If the spider decided to balloon, it altered its posture. (D) The spider rotated its body in the direction of the wind and assumed tiptoe posture.\

While the spider is standing on its blade of grass or leaf, it anchors itself to the substrate with a “drag line”, which is then passively severed after the spun “balloon” carries them away. The drag line not only anchors them firmly (they do this normally), but keeps them from blowing away before they’ve spun a sufficiently large parachute.

Below you can see the triangular shape of the balloon, spun on the tiptoe posture. The chute is several meters long and so light that it can take the spider long distances even with fairly gentle winds.

Three new facts about ballooning were uncovered. First, the crab spider does not evaluate the wind condition passively, but actively by raising 1 of its legs I. Second, this adult ballooner anchors its drag lines on the platform not only during its rafting takeoff but also during tiptoe takeoff. Third, the crab spider postures all its legs outward and stretched, when airborne, not only at the takeoff moment but also during the gliding phase.

One mystery discussed by the authors, and shown in the supplementary figure below, is that while ballooning the spiders keep their legs outstretched. That would seem to be aerodynamically inefficient. Wouldn’t it be better to curl up?

I have no answer here, but perhaps adjustment of body shape can help the spider “decide” where to land. It’s still not clear whether the spider has any say where it winds up, in terms of deciding where to settle, or just passively touches down when the wind abates. Clearly many spiders die when their balloons put them in the water or unfavorable habitat (of course spiders often have huge broods), but for this behavior to evolve by natural selection, the reproductive advantage of ballooning must exceed the costs of accidental death as well as the costs of staying put (getting eaten by your siblings, competing for food, etc.).

See how the keep their legs stretched out when flying?

Spiders’ posture in takeoff and flight. (A, B) An anchored line was found during a tiptoe takeoff. As soon as spiders were airborne, they stretched the legs outward. (C) To ensure the behavior of outstretched legs during flight, the pose of a spider was observed during its gliding phase. (D) The spider kept its legs outstretched.

There is a lot of information in the paper about the nature of the silk used to make the balloons, but I suspect you, like me, would find this less interesting. The coolest part is the description of how the spider does this, especially their testing of wind direction and speed by raising their legs into the air. That wasn’t known before, and I find it amazing.

Here’s a video, put out by the magazine Science, that describes the paper’s results. I’ve put it here at the end because if you watched it you might not want to read any further!

Yea, exactly what I thought. I’m sure this could be tested, with a dead, or model, spider, comparing splayed legs and balled up legs to see the way they spin, tumble, or not, in a wind tunnel captured with highspeed cameras. It might not solve this issue beyond a shadow of a doubt, after all, it may be the spider’s physical reaction to flying (and in their little spidey brains screeming “oh sh!t” as they lift off) but it would be nice to know either way.

Just a layman here, but it seems like if they are being carried by the breeze, they would be traveling at the same rate as the air and there would be no air resistance for the spread legs to effect. E.g., if you were in a river and were being swept along by the current, spreading your arms out would not slow you down, but it might help you maintain your orientation.

You may be right. I was thinking more about a difference between the spider’s drag and the parachute’s drag. But, as you say, if they are both going the same speed as the wind, then drag is not an issue. However, there is also gravity involved. I assume the spider weighs more than the parachute so would hang below and somewhat behind the parachute as both are swept along by the wind. I suppose this is more about stability than drag.

That’s a good analogy, and you’re correct. When you look at it in terms of Galilean Invariance you’ll realize that it’s in the spider’s interest to minimize its speed, because the speed that counts here is airspeed. Therefore, drag is the ticket. That’s what the parachute-web produces and what the spider contributes to slightly with its legs.

This isn’t quite right, because the spider has mass and is moving in a gravitational field.

AS I mentioned above, it appears that the spider is not trying to be carried horizontally by the wind (It doesn’t launch until the winds are minimal) but trying to go UP on thermals. The analogy is not with a person spearding his arms in a horizontally moving river, but rather of a person in a lake with lead boots. When he finds an upward current, he should spread his arms to help him go up instead of down.

Sensing thermals seems like a stretch for a spider though it probably can sense when it is going up or down. Instead, perhaps their triangular sail acts somewhat as a kite. Although a kite made only out of string wouldn’t work too well, when scaled down to a spider’s size the aerodynamics and fluid mechanics change dramatically. There are many things about flight at this scale that are still being discovered. We know the physics but not so much the ways in which creatures can take advantage of them.

It may be though that the outstretched legs enable the spider to turn its sail at an angle to the wind, which means it could go faster than the wind like a yacht. They appear to be talking about the parachute as triangular, which makes it more like a sail. The spider’s body would then be analogous to the boat, and the legs to the rudder.

Yes, something like that. While it is true that there’s no wind relative to a balloon floating in a constant wind, things are not so simple in a dynamic wind environment as usually exists near the ground.

A nice detail to admire is the anchor thread that breaks when the spider takes off. So the tensile strength of this could ensure that the balloon is in an optimal range of sizes. If it breaks too soon the spider might not be able to go very far because its balloon is too small. If it breaks too late, the ballon would be larger than needed and that would waste silk.

The video says their parachute is made up of about 60 threads of silk in a triangular form. But they don’t appear to do anything but shoot them out. I’d like to know how they spin 60 separate threads while keeping them all attached to their body, and how they make a triangular form without moving…

Spinnerets are a small bundle of short appendages at the rear of the abdomen. Each has a number of openings that produces silk, and I suppose there are 60 openings. This is surprising to me since I would have not guessed nearly that many.

Retired pilot, here. Outstretched legs make it more likely to snag a suitable substrate on contact and remain attached rather than bouncing around like a ping pong ball. And the added drag may help stabilize the whole assembly. A comparison of center of pressure compared to center of mass would answer the question. I have watched ticks fall from a perch and they always fall with legs spread, the better to catch something on the way down.

A grim side note: I had a friend who was an elevator mechanic. He told me that they were trained to, in the event of falling, hold their arms and legs out as far as possible and flail around because any time you hit something, it slowed you down and might save you.

The spider’s balloon is moving with the wind, so the spider’s velocity relative to the air is virtually zero. That means the spread legs are going to have no essential effect on drag.

As another poster noted, the spread legs may well be a stability mechanism. They would work a little like the pole used by high wire artists to maintain balance. Small twitches of the legs would change the spider’s center of gravity.

It seems that no one here has taken a hot-air balloon ride, or has seen the balloon scene in Around the World in 80 Days. David Niven’s hair was unruffled, and his wine glass didn’t blow out of the basket; all aboard were just riding the wind.

More seriously, this behavior has made spiders, with the notable exception of the mygalomorphs, truly cosmopolitan, and more efficient than many orders of winged insects, in colonizing remote islands and newly accessible habitats. I’ve been following certain [wingless] insects up the BC and Alaska coasts, and find rapid drop-offs in forest soil diversity away from ice refugia. Spider diversity, however, remains high everywhere in the recently glaciated regions.

Another note: A few times in my life, I’ve seen spectacular displays of gossamer — typically in warm, humid, nearly calm weather, with morning or light evening sunlight illuminating the silk of myriad used spider parachutes.

In other words, if the goal is to go up, they should maximize their resistance to the air, so that the rising air pushes them up. If they did not have much drag, they would be pulled down by gravity even if the air were rising.

Other than humans, I can’t think of another species that actively tests for wind speed and/or direction. I don’t know if even birds do this. Maybe birds figure it out once they’re aloft and don’t care to test the wind beforehand. Either way, it is very cool.

Yeah, I guess my observation is that birds don’t really “test” the wind. They just jump and go regardless. Whereas the spiders are “testing” to see if their kite/parachute will work. A bird knows their wings will work, so no need to test the wind before taking flight.

This might suggest a solution to the Google interview problem of how to get out of a blender if you are 1/2” tall – get up on the wall, hold your clothing like a sail, and when it turns on, the clothing will catch the air turbulence…